One of the most important limits in human information processing is the capacity of short term memory. The size of the memory span is correlated with IQ and is related to cognitive deficits found in psychotic and brain damaged individuals. In spite of its importance, no constant, noncircular measure of capacity in terms of items or chunks has emerged. Recently, several investigators have found that the memory span for a type of material is equal to the number of items that can be pronounced in approximately 1.5 seconds. Since this time seems to be the same for all types of material, a reasonable interpretation is that it is the mean duration of the memory trace. If so, the duration is no doubt a random variable, and we would be interested not only in its mean, but in its variance and density function. A means for estimating these is proposed. A variable that affects memory span can do so in two ways, either by affecting the pronunciation rate or by affecting the trace duration. Three variables known or suspected to influence memory span, namely noise, acoustic similarity and output modality will be investigated to see where their effects occur.